As an exhaust gas purification system for an internal combustion engine, there is a known system having an NOx selective catalytic reduction catalyst that is provided in an exhaust passage of the internal combustion engine to reduce NOx in the exhaust gas using ammonia as a reducing agent. In such an exhaust gas purification system, urea is supplied to the NOx selective catalytic reduction catalyst. Urea thus supplied is once adsorbed by the NOx selective catalytic reduction catalyst, and adsorbed urea is hydrolyzed to produce ammonia. Ammonia serves as a reducing agent to reduce NOx in the exhaust gas.
In some cases, a particulate filter (which will be simply referred to as “filter” hereinafter) that traps particulate matter (which will be referred to as “PM” hereinafter) in the exhaust gas is provided in the exhaust passage of the internal combustion engine. When this is the case, the temperature of the filter is sometimes raised to remove the PM trapped in the filter.
Japanese Patent Application Laid-Open No. 2004-270565 discloses a configuration in which an oxidation catalyst, a filter, a liquid injection nozzle for injecting urea-based liquid into the exhaust gas, an NOx selective catalytic reduction catalyst, and a catalyst for preventing slippage of ammonia are arranged in order from the upstream in the exhaust passage. In this configuration disclosed in Japanese Patent Application Laid-Open No. 2004-270565, an EGR pipe and an EGR valve are further provided. The liquid injection nozzle and the EGR valve are controlled based on the operation state of the internal combustion engine.